Hi Luk
For pure slalom, its got be the iS127/9.5m. At 80kg you will easy control this combo in 9-15k and it will be powered to very powered up (for 80kg rider). Some will say a 9.5m is too much for an 80kg in 15k, but this combo likes to be well powered up.

I feel that the iS117/8.6m is too close to your iS107/7.8m and would just be a lot of expensive for just a small benefit in light winds, so better to go all the way get a proper light wind slalom combo.

You could also consider the iS117/9.5m. I think Steve Alan uses this combo as his light wind combo and he is 80kg ish. Other light/medium PWA guy's are using the iS117/8.6m combo as their light wind set up, but I guess that part off their choosing decision is based on easier/cheaper travelling to and from events. Plus they have uber efficient early planning skills.

If you go with the iS117/9.5m combo, it could be worth hanging on for the 2014 model. I see the 2014 iS107 is going to be 70cm wide, so maybe the iS117 is going to be more powerful than the 2013, just a guess